Ink jetting devices are well known in the art. Ink jet printing systems are generally of two types: continuous stream and drop-on-demand. In continuous stream ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed, causing it to break up into droplets at a fixed distance from the orifice. At the break-up point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field that adjusts the trajectory of each droplet in order to direct it to a gutter for recirculation or a specific location on a recording medium. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium. There are generally three types of drop-on-demand ink jet systems. One type of drop-on-demand system is a piezoelectric device that has as its major components an ink filled channel or passageway having a nozzle on one end and a piezoelectric transducer near the other end to produce pressure pulses. Another type of drop-on-demand system is known as acoustic ink printing. As is known, an acoustic beam exerts a radiation pressure against objects upon which it impinges. Thus, when an acoustic beam impinges on a free surface (i.e., liquid/air interface) of a pool of liquid from beneath, the radiation pressure which it exerts against the surface of the pool may reach a sufficiently high level to release individual droplets of liquid from the pool, despite the restraining force of surface tension. Focusing the beam on or near the surface of the pool intensifies the radiation pressure it exerts for a given amount of input power. Still another type of drop-on-demand system is known as thermal ink jet, or bubble jet, and produces high velocity droplets. The major components of this type of drop-on-demand system are an ink filled channel having a nozzle on one end and a heat generating resistor near the nozzle. Printing signals representing digital information originate an electric current pulse in a resistive layer within each ink passageway near the orifice or nozzle, causing the ink vehicle (usually water) in the immediate vicinity to vaporize almost instantaneously and create a bubble. The ink at the orifice is forced out as a propelled droplet as the bubble expands.
In a typical design of a piezoelectric ink jet device, the image is applied by jetting appropriately colored inks during four to eighteen rotations (incremental movements) of a substrate such as an image receiving member or intermediate transfer member with respect to the ink jetting head, i.e., there is a small translation of the printhead with respect to the substrate in between each rotation. This approach simplifies the printhead design, and the small movements ensure good droplet registration. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the recording substrate, either directly or via an intermediate heated transfer belt or drum, they quickly solidify to form a predetermined pattern of solidified ink drops.
Ink jet printing processes may employ inks that are solid at room temperature and liquid at elevated temperatures. Such inks may be referred to as hot melt inks or phase change inks. In thermal ink jet printing processes employing hot melt inks, the solid ink is melted by the heater in the printing apparatus and utilized (i.e., jetted) as a liquid in a manner similar to that of conventional thermal ink jet printing. Upon contact with the printing substrate, the molten ink solidifies rapidly, enabling the colorant to substantially remain on the surface of the substrate instead of being carried into the substrate (for example, paper) by capillary action, thereby enabling higher print density than is generally obtained with liquid inks. Advantages of a phase change ink in ink jet printing are thus elimination of potential spillage of the ink during handling, a wide range of print density and quality, minimal paper cockle or distortion, and enablement of indefinite periods of nonprinting without the danger of nozzle clogging, even without capping the nozzles.
Current phase change inks often comprise custom dye colorants. These custom dyes are very expensive. It is desired to replace custom dye colorants with less expensive colorants. Pigments are typically much less expensive than dye colorants. In addition, pigments can offer improved colorfastness over dyes, reduce or eliminate migration issues, and improve ink robustness characteristics.
Pigmented phase change ink compositions that include various dispersants are also known. However, the use of polymeric dispersants is not favored in some phase change inks for a variety of reasons. The problems caused by the use of polymeric dispersants include a negative effect on rheological properties of the ink, such as non-Newtonian behavior and an increase in viscosity.
Pigment particles in the ink must be properly dispersed and stabilized such that the ink can be reliably jetted without the clogging of the printheads by the pigment particles. Polymeric dispersants in phase change inks also affect drop formation, because polymers will tend to form filaments which affect the formation of small drop sizes. Most of the commercially available dispersants were designed for aqueous based and solvent based ink systems and are not compatible with hydrophobic wax based inks. Many of the commercially available compounds that can effect dispersion of pigments in low polarity inks (usually solvent-based) are liquids or pastes and are not designed to chemically withstand the excessive temperatures in the printer (115 to 120° C.) for long periods of time. Furthermore, the use of polymers in solid ink is not favored for the following reasons: a) they have a negative impact on rheological properties producing non-Newtonian behavior and an increase in viscosity, b) they affect drop formation during jetting, polymers will tend to form filaments which might affect the formation of small drop sizes.
While known compositions and processes are suitable for their intended purposes, a need remains for pigment stabilizing resinous compounds that are chemically stable, are compatible with the phase change ink formulation and that can provide stabilization of pigment particles in phase change inks over long periods of time at high temperatures. There is further a need for an improved colored phase change ink composition. For example, there remains a need for phase change inks with pigment colorants where the pigment particles are stable and well dispersed in the ink. There remains a need for pigmented phase change inks with improved image quality, improved light fastness, and reduced show through. A need also remains for pigmented phase change inks where the colorants have reduced agglomeration and settling in the ink when the ink is exposed to high temperatures for prolonged periods. A need also remains for pigmented phase change inks with reduced clogging of the jets in the printhead.